Is there a stranger film-maker on the planet than Spike Jonze? How many other directors would have followed up a mould-breaking movie like Being John Malkovich with a 30-minute home video about a washed-up rapper? What's Up Fatlip? is a mini-documentary about the making of the video for the single of the same name, by former Pharcyde MC Derrick "Fatlip" Stewart. It takes the form of a series of cut-together out-takes, but as this is a Spike Jonze production, you can never be sure what really happened and what didn't. The film details the life of a pitiful loser: the hand-held camera captures Fatlip fleeing from a chihuahua, riding a bike with a child's seat, and failing pathetically as a kids-party clown. He also talks about how his friends think he's gay because he had sex with a transvestite (as commemorated in the Pharcyde track Oh Shit).

Fatlip, a founder member of Pharcyde, parted ways with the rest of the west coast hip-hop collective a couple of years ago, and there's a near-total blurring of truth and fiction here. But that is Jonze's way: he presents a totally deadpan face to the world, however outlandish the material he's dealing with.

Remember the shenanigans surrounding the best-directed video prize at the 1999 MTV video awards? When Praise You was announced, a gaggle of men and women in tracksuits swarmed out of the wings to join Fatboy Slim on stage. "Ladies and gentlemen," intoned Fatboy, "Richard Couffey!" A gawky, bespectacled figure emerged from the onstage throng, clutching a gym bag. "The Torrance Community Dance Group and I have been together for seven years," gushed the figure in a choir robe as he hoisted the statuette aloft, "and this is by far the most amazing thing that has ever happened to us!" It was clear even then that Richard Couffey never existed, nor the Torrance Community Dance Group - and that Jonze, by putting himself in a fake homemade film of a local dance group performing outside a shopping mall film theatre, was playing an elaborate practical joke.

Of course, Jonze doesn't exist either. The man's real name is Adam Spiegel, and he appears to bring the Jonze character out of the box - along with a host of others - when he feels like it. In a recent BBC documentary, Spiegel/Jonze presented himself as a Corvette-driving loudmouth. During the Praise You shoot, according to Norman Cook (a mere amateur in the pseudonym stakes), he remained in character as Couffey. Cook recalls being unable to actually talk to him.

As a film-maker, Jonze developed his identity-shuffling skills in music video, where the reduced demands on comprehensibility and coherence gave him leeway to experiment. He put himself front and centre in Praise You, but it was another Fatboy Slim tune, Weapon of Choice, that was his pièce de résistance of character manipulation. The simple idea of Christopher Walken's soft-shoe freak-out was enough to get Weapon of Choice voted the greatest video of all time in a music-industry poll last month on cable channel VH1.

Jonze did a similar image flip a few years earlier with Björk's Busby Berkeley routine for It's Oh So Quiet. The runner-up as best video in the VH1 poll was another of Jonze's, the Beastie Boys' Sabotage, in which the Beasties hammed it up as cops from a 1970s TV show. But strangest and most straight-faced of all has to be the Daft Punk video, Da Funk, which follows a man with a dog's head around New York.

As he has gone on, Jonze has delved further into the detritus of celebrity culture, with Being John Malkovich now appearing as an inevitable conclusion. His follow-up feature, also scripted by Charlie "Malkovich" Kaufman, promises to take things even further. Advance information tells us that the movie, entitled Adaptation, features a Kaufman clone (played by Nicolas Cage) wrestling with an adaptation of a book called The Orchid Thief by Susan Orlean - which in turn tells the true-life story of a man who illegally cloned orchids to sell to plant collectors. Not only does the script lug in a character called Robert McKee (yes, the script-writing seminar guru), but John Cusack and Catherine Keener are also called on to re-create their characters from Being John Malkovich.

This from the same man that gave the world Jackass. The other side of the Jonze prankster persona returned to his MTV roots as writer and director of the show, a cavalcade of idiotic stunts and exhibitionism (jousting on dirt bikes, sewage diving, hurling flour bombs at sleeping people, etc). Again it demonstrates his ability to persuade stars to perform the most unexpected roles. For one sketch, Brad Pitt, of all people, dressed in a monkey suit, spent the night creating havoc in a supermarket car park by jumping in and out of shopping trolleys. On another occasion he allowed Jackass regulars to kidnap him from the queue at a hotdog stand on a Los Angeles street, leaving bystanders stunned. Jonze can often be heard off camera egging on participants ("Hit him in the nuts!") as the cast perform random acts of mindlessness to entertain their audience and each other.

Never before has arrested development looked like so much fun.

What's Up Fatlip? is part of onedotzero6, a trio of films about LA life, at the ICA, London SW1, at 7pm tomorrow and until Sunday May 26. Box office: 020-7930 3647.